This invention relates to a method and apparatus for grinding a trochoidal curved surface comprising the inner curved surface of a rotor housing of a rotary engine, and more particularly to a method and apparatus wherein a worktable supporting a workpiece, for example, a rotor housing, is profile controlled by means of a cam having a profile corresponding to the trochoidal curved surface to be worked and a solid roller cooperating with the cam, the rotation and revolution necessary for the grinding operation are inparted to the worktable and the workpiece is ground while the axis of the grinding wheel is maintained at a definite position.
Although various methods and apparatus have been proposed to grind a trochoidal curved surface comprising the inner curved surface of a rotor housing of a rotary engine, this invention relates to a method and apparatus of the class wherein a cam having a profile corresponding to the trochoidal curved surface to be work is used, and the attitude of a worktable on which a rotor housing or a workpiece is mounted is controlled by the cooperation of the cam and a roller, whereby to grind the workpiece.
One example of the prior art methods of grinding the workpiece by using a cam will first be described with reference to FIGS. 1 and 2 of the accompanying drawings. As shown in FIG. 2, a downwardly projecting master cam MC provided with an inner master profile M having the same profile as an internal curved surface to be worked and an outer drive profile N formed by an envelope spaced an equal distance in the radial direction from the inner master profile M is formed on the lower surface of a worktable T which supports a rotor housing of a rotary engine, for example, that is a workpiece, and on a stationary portion of the machine, not shown, are rotatably mounted a master roller MR which is urged against the master profile M of the master cam MC, and a drive roller DR which is urged against the drive profile N so as to clamp the master cam MC between it and the master roller MR. When the drive roller DR is driven by driving means, such as an electric motor, not shown, the worktable T is rotated by the rolling contact between the drive profile N and the drive roller DR so that the work piece mounted on the table T is ground by a grinding wheel, not shown, in a manner as will be described later. A secondary cam groove SC is formed on the periphery of the master cam MC to receive a secondary roller SR rotatably supported by the stationary portion so as to control the attitude of the master cam MC. Thus, the table T is rotated while its attitude is being controlled by the cooperation of master cam MC and the rollers such that normal line to the master profile M and passing through a contacting point P between the master roller MR and the master cam MC always alignes with a line OM-OD interconnecting the center of rotation OM of the master roller MR and the center of rotation OD of the drive roller DR. On the other hand, a grinding wheel G is supported above the table T with its axis G' positioned on the line OM-OD and its periphery passed through the contacting point P so as to work the inner surface of the workpiece. In this manner, it is possible to form by grinding operation a curved surface having the same profile as the master profile M. This method requires to use a master cam and a secondary cam and since the accuracies of these cams influence directly the working accuracy, the accuracies of the cams should be extremely high. According to the present day engineering level, machining of cams of such high accuracies is difficult. Especially, it is extremely difficult to finish the master profile M of the master cam MC and the secondary cam groove SC with the desired high accuracies. Further, according to this method, since a rotating torque is imparted to the table T by the rolling contact between the drive roller DR and the master cam MC, in order to impart a large driving torque it is necessary to increase the contact pressure, hence the frictional force between the drive roller DR and the drive profile N. For this reason, the materials for constructing the cam and roller are limited by such physical characteristics as the frictional characteristic, fatigue characteristic, strength, etc. of the materials. In order to decrease the contact pressure it is necessary to increase the size of the cam or roller, or to subject a special material to a special heat treatment or surface treatment. Yet, reliable products are still difficult to obtain. To obviate this difficulty, it has been necessary to use a coupling means that can transmit the driving power from the source of drive to the worktable without relying upon the power transmission system utilizing rolling contact.
Further, although this method is advantageous in that its principle is simple and that even when the diameter of the grinding wheel varies it is still possible to finish an accurately ground profile by merely adjusting the amount of feed of the grinding wheel, as the angular velocity of the worktable varies positively or negatively in accordance with the irregularities of the curved surface to be worked it is impossible to rotate the worktable at a high speed so that the efficiency of the grinding operation is not high.